Along with development of the economy and society, the energy crisis becomes increasingly prominent and the global environment gradually deteriorates. Therefore, developing and using clean alternative energy has become an important goal in the energy industry. In the wake of the continuous development of new energy power generation, energy storage, and new energy automobile industries, as a core energy control apparatus, a converter becomes one of the key factors in the application of clean energy. The converter is an essential unit for implementing transfer of renewable energy, such as solar photovoltaic energy, to a power grid.
Among various types of converters, three-phase converters are one type of most widely applied converters, and are used to connect a three-phase alternating current electrical power system and a direct current electrical power system and implement energy transfer between the two systems. Energy transfer is further distinguished into two working statuses, rectification and inversion, according to different energy flow directions. Energy transfer from the direct current system to the alternating current system is referred to as inversion, and energy transfer from the alternating current system to the direct current system is referred to as rectification.
Therefore, in most application scenarios, rectification and inversion can be implemented using a same system. A typical system structure of the system is shown in FIG. 1. A three-phase converter is connected between a direct current system and an alternating current system, and includes a switching network, a filter, and a controller that controls the switching network.
Conversion efficiency and electric energy quality are two key technical indicators of a three-phase converter. However, a modulation method directly affects a connectivity status of switching devices, and therefore, is one of key factors that affect the conversion efficiency and electric energy quality of the three-phase converter.
A commonly used modulation method is pulse-width modulation (PWM), that is, a width of a driving pulse of each device on a switching network is controlled. A most direct implementation is to compare a carrier and a modulated wave, and control a connectivity status of a switching device using a result of the comparison.
PWM may be further categorized into continuous PWM (CPWM) and discontinuous PWM (DPWM). The CPWM means that there is always a switching action in each phase bridge arm in each switching period. Common methods include sinusoidal PWM (SPWM), space vector PWM (SVPWM), and third-harmonic-injection PWM (THIPWM). The DPWM means that a phase bridge arm of a converter is clamped on a positive direct current bus or a negative direct current bus in a specific switching period, and a switching device of this phase is always connected or always disconnected in a clamping interval and there is no switching action. Common DPWM modulation methods include DPWM0, DPWM1, DPWM2, DPWM3, DPWMMAX, DPWMMIN, GDPWM, and so on.
Compared with the CPWM, the DPWM has a smaller quantity of switching times. Therefore, a switching loss is relatively small, and a consequential benefit is that efficiency of a converter can be improved.
In specific implementation, a DPWM modulated wave may be implemented by superposing an equivalent common-mode component on a CPWM modulated wave. For example, a DPWM modulated waveform and an SPWM modulated waveform in an industrial frequency period (50 hertz (Hz)) are compared, as shown in FIG. 2. A difference between the two modulated waves is a common-mode signal waveform shown in FIG. 2.
The DPWM modulated wave may be equivalent to a sum of SPWM wave and a common-mode signal. Therefore, an output characteristic of DPWM wave is affected by both an output characteristic of SPWM and an output characteristic of the common-mode signal, and an extra common-mode voltage source is formed at a bridge arm port of a converter. The common-mode voltage source and a common-mode loop in a system interact with each other, affecting system performance.
As shown in FIG. 3, in a typical three-level LCL filter converter to reduce a common-mode current transferred to an alternating current system by the converter, a middle point of capacitors of the LCL filter is connected to a middle point of capacitors on a direct current bus, to form a new common-mode loop. This is equivalent to splitting a common-mode current generated by the converter such that most of the common-mode current is directed back to a direct current side of the converter using a connection loop of the LCL filter. However, a series connection of an inductor and a capacitor exists in the connection loop, and there is a natural series resonant frequency. If the frequency overlaps a frequency range of a common-mode voltage source generated because of DPWM modulation, series resonance is generated, causing sharp fluctuation of the common-mode current, thereby affecting system stability.